Over 300 nuclear reactors have been proposed, of which 136 will be in China, 24 in the USA and 23 in Russia. India’s massively delayed nuclear power programme will see a resurrection after Électricité de France (EDF), the world’s biggest electricity company, agreed to build six nuclear plants in the country. The Indian Jaitapur project is expected to become the world’s biggest nuclear contract and one of the world’s largest nuclear sites. The 10,000MW project will have six reactors of 1,650MW each.
Fossil fuel-powered generators are still expected to play a major part in the ever-increasing global demand for electricity. An estimated 1,000GW of new coal-fired power stations are planned over the next 20 years. Half of these will be in China, but with significant programmes in South Africa and India. Most of the global coal-fired installations are old and maintenance programmes will need to be implemented coupled with the introduction of CCS (carbon capture and storage) retrofitting.
The planned surge in new electricity power generation plant and refits across the world brings with it a demand for improvements in welding technology. This demand will be met by innovative developments in welding equipment to ensure consistently better quality joints, many of which are in the safety-critical class.
High-pressure materials present challenges in welding practices
All the programmes involve extensive fabrication of high-pressure steel pipes and tubes, the welding of which presents particular challenges.
The high pressures and temperatures used in steam generation circuits necessitate the use of creep resistant steels such as those based on chromium/molybdenum/vanadium alloys. These materials provide improved oxidation and corrosion resistance together with high strength and are widely used in both fossil fuel and nuclear power plants.
The demand for quality in all these safety critical joints is reflected in the stringent regulations laid down in welding procedures. Nevertheless, some welding practices can result in considerable reduction both in corrosion resistance and mechanical strength.
Welding of high-pressure steam pipe
Some engineering alloys are prone to cracking during welding. Industry sectors having to overcome this problem are principally in the power generation sector and include low and medium alloy steels that have been specially developed for their high strength. Metallurgists have learned that heating the joint prior to and after welding (pre-heating and post-heating) can reduce the sensitivity to cracking quite significantly. It involves temperatures in the region of 200°C, although this may be much higher for certain materials.
Welding is one process that is widely used during manufacture. This affects the microstructure. Preheating, maintaining inter-pass temperatures and post-weld heat treatment procedures are very critical for these creep-resistant alloys. Failure to follow the procedures can result in catastrophic failures in service.The preferred welding procedures in this type of fabrication are GTAW and GMAW and these offer protection of the exposed upper fusion zone. The joint around the underbead, however, needs to be protected by purging – the protection of exposed metal by applying a local inert gas atmosphere.
Meeting the requirements of inert gas purging when temperatures exceeding 200ºC are involved necessitates the use of purge systems capable of withstanding these temperatures throughout the heating and welding cycles. Typical thermal cycles can exceed two hours and it may be necessary to maintain the purge system in place throughout.
Gas purging system requirements
Specially engineered purge products have been designed over the past five years that are capable of withstanding the temperatures involved whilst at the same time maintaining adequate gas sealing characteristics. They are also rugged enough to survive multiple-use applications. These products are manufactured from thermally stable engineering polymers and can be provided with advanced gas valve control systems.
Few manufacturers are able to supply weld purging systems that can be used at the high temperatures prevailing during pre- and post-heating but some commercially available systems have been designed specifically to meet the requirements.
However, it is clear that many companies still employ paper, cardboard and polystyrene foam as dam materials. These are prone to outgassing during use, are difficult to insert and remove, and may even ignite at the prevailing temperatures.
Weld gas purging techniques
The most effective devices are those based on connected inflatable dams.
The inert gas input can be programmed to control gas flow and pressure during inflation and purging and once placed in position require little more input from an operator. The dams are fabricated using advanced engineering polymers and are thus suitable for use where elimination of contamination is essential.
Purge gas oxygen content can be controlled by using an oxygen monitor. These instruments not only measure oxygen levels but also inhibit welding if the level is above that set by the operator. Recording and analysing software provides information for quality control purposes.
Conclusion
Even very low oxygen concentrations in weld gases can give rise to discolouration, loss of corrosion resistance and reduction in mechanical strength. Controlling oxygen level in purge gas can be effected simply and efficiently using contemporary integrated purge systems.
Dr Michael Fletcher wrote this article on behalf of Huntingdon Fusion Technologies
